Fluorescence tomography (FT) and diffuse optical tomography (DOT) are imaging techniques by which tomographic representations of optical properties of a subject, i.e. of its biological tissue, can be obtained in vivo. These techniques are mostly, but not exclusively, applied for preclinical small animal imaging. They may also be applied for clinical imaging including screening for breast cancer and functional imaging of the brain.
In the case of diffuse optical tomography (DOT) the obtained optical properties are usually the absorption and scattering coefficients or similar of the biological tissue of the examined subject, the coefficients relating to morphological and functional properties of the biological tissue such as vascularisation and perfusion.
In the case of fluorescence tomography (FT) a three-dimensional map of fluorescence emission is obtained by reconstruction. The map may for instance represent the distribution of fluorescent molecular markers that have been administered to the examined subject. The marker molecules bind to or are activated by their target molecules in the examined biological tissue. The measured distribution of the activated molecular markers thus represents the concentration of specific target molecules. Application of fluorescence tomography is widespread in oncology, where fluorescence tomography can for instance be used to monitor the expression level of oncogenes in cancerous tissue in mice used as mammalian animal models during research and investigation of human cancer. Fluorescence tomography (FT) is also known as fluorescence enhanced diffuse optical tomography (fDOT) or as fluorescence molecular tomography (FMT).
In diffuse optical tomography (DOT), fluorescence tomography (FT) and in fluorescence imaging (FI) (e.g. small animal FI) measurements are obtained by illuminating a part of the surface of the examined subject with a light beam and by detecting and quantifying light re-emitted from a part of the surface of the subject. Reconstruction algorithms are used to reconstruct an image or images of the subject from the measured data. Minor variations exist in the way illumination and detection are performed and in the reconstruction algorithms employed.
The illumination of the subject is typically, but not necessarily, point-like and is achieved with monochromatic light that preferably has a wavelength in the near-infrared (NIR) spectrum. The detection and measurement of the light that is (re-)emitted from the subject is realized by imaging the re-emitting part of the examined subject onto a detection unit with an electro-optical sensor such as a CCD (charge-coupled device) sensor or a CMOS (complementary metal oxide semiconductor) sensor. This imaging may be repeated for different illuminated points on the subject.
In the case of diffuse optical tomography (DOT) the light that impinges onto the biological tissue propagates within/through the biological tissue of the subject and emerges from a part of the surface of the biological tissue, with the emerging light being measured. Fluorescence tomography (FT) often also comprises measurement of the illuminating/excitation light in addition to the measurement of the fluorescence (signal) intensity that arises at the surface of the examined subject. Measurement of the fluorescence intensity is typically realized by using appropriate filters in the detection path that efficiently block the wavelength of the illumination/excitation light but that are transparent at least for a part of the emitted fluorescence spectrum.
Biological tissue has, however, the property of strongly scattering visible and near-infrared (NIR) light. Hence, each photon that penetrates the biological tissue and propagates through it undergoes a large amount of scattering events before it is either absorbed by the biological tissue or re-emitted from it. The light propagation through and within the biological tissue is often modelled as diffusion process with the corresponding diffusion equation, although other models are sometimes used. Accordingly, the reconstruction algorithms used in diffuse optical tomography (DOT) and fluorescence tomography (FT) are usually based on the diffusion equation. The diffusion equation can be solved within an FEM (finite element method) or a FVM (finite volume method) framework with boundary data being given by the measured surface images of the examined subject, the solution of the diffusion equation yielding the distribution of the scattering and absorption coefficients or the fluorophore concentration.
A common arrangement of the illumination unit and the detection unit in an optical imaging system such as for DOT, FT or FI is such that illumination and detection are realized on opposite sides of the examined subject. An optical imaging system realized in such a way is also called transillumination system as the light transmitted through the subject is detected. FIG. 1 schematically depicts a possible implementation of such a transillumination DOT or FT system/optical imaging system 1. The illumination unit is realized by a laser 2 as light source that is pointed at the examined subject 3 such that its beam impinges onto the surface of the examined subject 3. FIG. 1 shows a mouse as exemplary subject. The optical imaging system 1 is, however, not limited to imaging mice or even biological specimen. The detection unit comprises a camera 4, a lens (not shown) and a rotatable set of filters 5 (also referred to as filter changer). The lens is arranged between the camera 4 and the set of filter 5. The set of filters 5 can be rotated such that different filters can be positioned in front of the lens depending on the particular application. The illumination unit 2 and the detection unit 4, 5 are mounted opposite each other on a rotatable platform/gantry 6, wherein the platform 6 has a central opening 7 with a dedicated holder 8 for receiving the subject 3 to be examined. The illumination unit 2 and the detection unit 4, 5 are arranged on opposite sides of the subject 3/the central opening 7. The platform 6 can be rotated as indicated by the arrow 9 such that transillumination of the subject 3 (and thus illumination and detection) can be performed at various angles with respect to the subject 3. Accordingly, an optical imaging system as depicted in FIG. 1 is also referred to as system for 360°-projection free-space fluorescence tomography. The holder 8 onto which the subject 3 is placed can be moved/shifted along the rotation axis 10 (translatory movement) of the platform 6 such that transillumination (and thus illumination and detection) can be performed at various positions of the subject 3 along the rotation axis 10.
Variations of the optical imaging system shown in FIG. 1 are described in the literature, including systems where the examined subject can be rotated and/or the illumination unit and the detection unit can be translatorily moved. Furthermore, optical imaging systems are known that provide further degrees of freedom for scanning the laser beam/the light (re-)emerging from the subject.
In an optical imaging system for transillumination (as depicted in FIG. 1), e.g. in systems for FT, DOT or FI (for example small animal FI) that use transillumination, light has to traverse a relatively large volume of biological tissue before a signal is emitted from the subject for detection by the detection unit at the side of the subject that is opposite to the entry point of the light beam/illuminating light. As mentioned above, biological tissue strongly scatters, reflects and absorbs light. Thus light propagation in biological tissue typically has a diffusive nature and most of the injected photons of the illuminating light become absorbed or diffusively reflected by the biological tissue, resulting in a very low measured signal (which is typically radiant power or radiant flux). The power of the part of the illuminating light that is diffusely reflected at the illuminated surface area of the subject (the reflected light) is typically of orders of magnitude higher than that of the signal to be measured. Furthermore, this reflected light (also called stray light or spurious stray light) can reach the detection unit via secondary reflections. The power of this spurious stray light can easily be high enough to saturate the detection unit or simply to lead to false measurement results. However, in particular in so called 360°-projection free-space fluorescence tomography systems as exemplarily depicted in FIG. 1 it is difficult to protect the detection unit from such stray light (also called background signals or background noise). To shield the subject to be examined from ambient light, the subject is enclosed in a light-tight, opaque chamber, whereby ambient light is to be distinguished from stray light.
In conventional FT, DOT or FI systems (e.g. small animal FI systems) planar optical imaging systems are used that do not allow for rotation of the illuminating unit and the detection unit around the subject. In these conventional planar optical imaging systems transillumination is performed from below the subject and detection takes place from above the subject. Light-shielding structures can be arranged very close to or even in contact with the subject's body. The subject, e.g. an animal, can for example be placed on a black surface that serves as shielding, the black surface having a window just below the subject's body to allow illumination to pass through the window and to impinge on the subject. Different implementations of relevant commercial planar FT and FI imaging systems are reviewed and discussed in F. Leblond et al., “Pre-clinical whole-body fluorescence imaging: Review of instruments, methods and applications”, Journal of Photochemistry and Photobiology B: Biology 98 (2010), pp. 77-94.
The above-described light-shielding solution that is employed for planar optical imaging systems is, however, not appropriate for optical imaging systems that provide a scanner function, where the detection and the illumination units rotate around the subject to perform measurements at different angles (as in the 360°-projection free-space fluorescence tomography system depicted in FIG. 1). The use of light-absorbing surfaces (for example through surface blackening) may alleviate the stray light problem, but does in many cases not lead to a sufficient reduction of stray light. Optical imaging systems that allow for rotation are, however, superior to planar optical imaging systems as they allow to image the subject from different angles and to thus acquire a much higher amount of relevant measurement data leading to an improvement in the reconstruction quality.
A 360°-projection free-space fluorescence tomography system, wherein the illumination unit and the detection unit rotate relatively to a subject animal, is discussed in combination with an X-ray CT (computer tomography) scanner in R. B. Schulz et al., “Hybrid Fluorescence Tomography/X-ray Tomography improves reconstruction quality”, Proceedings of SPIE-OSA Biomedical Optics, SPIE Vol. 73720 (2009), 73700H-1-73700H-4. The stray light problem is not addressed.
In practice some light-absorbing surfaces positioned behind the subject to be examined will be in the field of view of the detection unit. In existing rotating DOT and FT systems these surfaces will then be illuminated by light reflected from the surface of the examined subject, i.e. by stray light. Intensities occurring at these surfaces can easily reach intensities that are high enough to lead to a saturation of the detection unit and can thus disturb the measurements of the (re-)emitted/transmitted light or of the arising fluorescence intensities. Moreover, the light that is reflected from the examined subject (i.e. the stray light) can bias the measurement results by additionally illuminating parts of the subject through multiple reflections. Such stray light problems can limit the performance of an optical imaging system due to low signal strength of the measured/detected signal, even if the significant surfaces of the system and light shielding structures which protrude into the field of view of the detection unit are blackened to enhance their light absorbance characteristics and light filters are used.
Patent document U.S. Pat. No. 6,462,889 B1 discloses a supersonic missile that comprises an optical system. Baffles are employed to reduce the effects of stray light. The missile does not comprise an active light source/illumination source and can not be used to image/scan a subject arranged inside the missile.